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Ask Joe Powder

"Ask Joe Powder"
"Ask Joe Powder" is a question and answer column authored by Kevin Biller of the Powder Coating Research Group. Mr. Biller has over 30 years experience formulating and manufacturing powder coatings. He welcomes your questions regarding powder coating technology. Please write to: askjoepowder@yahoo.com 

Kevin Biller
The Powder Coating Research Group
15 W. Cherry Street, 3rd Floor
Columbus, OH 43215
614-354-11988
www.powdercoatingresearch.com 


I’m having a big problem with my powder coating chipping off my customer’s parts. These are automotive seat assembly brackets. First we media blast the parts with aluminum oxide to remove any rust, then wipe clean with acetone. Next we apply 2.0 to 3.0 mils of a zinc-based epoxy powder primer, curing per our supplier’s recommended bake conditions (20 minutes at 355°F) then topcoating with a standard bake polyester at about 3.0 mils. The polyester is cured per the 20 minutes at 375°F bake recommended on the technical data sheet provided by the powder manufacturer. The loss of adhesion occurs between the epoxy primer and the polyester topcoat. Could we be applying the powder too thick? Is the powder undercured? Should we bake the powder longer or at a higher temperature?
Your advice will be greatly appreciated.

Jamie Wells – RPM Powder Coatings, Columbus, Ohio

Thank you for the question. By applying a two coat system you really are giving your customer a “Cadillac” coating that should beyond the life of the vehicle. When marrying two of more coatings on a substrate there are certain procedures that can be used to ensure excellent adhesion and performance.

Let’s go through your process. Your metal preparation appears to be sound. Media blasting can provide an excellent surface for an epoxy primer. It is important to keep your media clean and to thoroughly blast the entire surface. A subsequent acetone wipe is a good idea also as long as you use clean, lint- free wipers.

You mentioned that you are using a zinc-rich epoxy primer. This technology can provide outstanding corrosion protection of ferrous substrates. It is important to note that epoxies are incredibly durable and quite hard when completely cured. Because of this it is wise to under-bake an epoxy based powder if it is being used as a primer. By under-curing the epoxy primer you are leaving a softer surface for the polyester topcoat to “bite” into. Furthermore under-curing provides a few unreacted functional
groups that can be chemically bound by the polyester. These unreacted groups are compatible with the carboxyl functional groups in the polyester and provide a much more intimate bond between the primer and topcoat.

I would recommend under-curing the primer by about 25°F (about 15°C) or for 50% of the recommended time at temperature. I would then apply the polyester topcoat and ensure that it receives the full recommended bake. It is also important to make sure the topcoat cures at a higher temperature than the primer. Otherwise you risk the possibility of undercuring the entire coating system.

Good luck with your project.
Best regards,

Joe Powder
Please forward any questions or comments to askjoepowder@yahoo.com.